1. Field of Invention
The present invention relates to a static memory. More particularly, the present invention relates to a magneto resistive random access memory (MRAM).
2. Description of Related Art
MRAM is a type of non-volatile memory with fast programming time and high density. A MRAM cell of giant magneto resistance (GMR) type has two ferromagnetic layers separated by a nonmagnetic conducting layer. Information is stored as directions of magnetization vectors in the two ferromagnetic layers.
The resistance of the nonmagnetic layer between the two ferromagnetic layers indicates a minimum value when the magnetization vectors of the two ferromagnetic layers point in substantially the same direction. On the other hand, the resistance of the nonmagnetic layer between the two ferromagnetic layers indicates a maximum value when the magnetization vectors of the two ferromagnetic layers point in substantially opposite directions. Accordingly, a detection of changes in resistance allows information being stored in the MRAM cell.
In conventional MRAM process, metal plugs used to connect a diffusion metal layer under a layer of GMR memory bits and a power plane metal over the layer of GMR memory bits have high contact resistance due to the small size and high aspect ratio of the metal plugs. Hence, there is a need to develop a low-resistance metal plug to connect the diffusion metal layer and the power plane metal.